Robert E. Rolley

Host culling as an adaptive management tool for chronic wasting disease in white-tailed deer: a modelling study

Summary 1 Emerging wildlife diseases pose a significant threat to natural and human systems. Because of real or perceived risks of delayed actions, disease management strategies such as culling are often implemented before thorough scientific knowledge of disease dynamics is available. Adaptive management is a valuable approach in addressing the uncertainty and complexity associated with wildlife disease problems and can be facilitated by using a formal model.2 We developed a multi‐state computer simulation model using age, sex, infection‐stage, and seasonality as a tool for scientific learning and managing chronic wasting disease (CWD) in white‐tailed deer Odocoileus virginianus. Our matrix model used disease transmission parameters based on data collected through disease management activities. We used this model to evaluate management issues on density‐ (DD) and frequency‐dependent (FD) transmission, time since disease introduction, and deer culling on the demographics, epizootiology, and management of CWD.3 Both DD and FD models fit the Wisconsin data for a harvested white‐tailed deer population, but FD was slightly better. Time since disease introduction was estimated as 36 (95% CI, 24–50) and 188 (41–>200) years for DD and FD transmission, respectively. Deer harvest using intermediate to high non‐selective rates can be used to reduce uncertainty between DD and FD transmission and improve our prediction of long‐term epidemic patterns and host population impacts. A higher harvest rate allows earlier detection of these differences, but substantially reduces deer abundance.4 Results showed that CWD has spread slowly within Wisconsin deer populations, and therefore, epidemics and disease management are expected to last for decades. Non‐hunted deer populations can develop and sustain a high level of infection, generating a substantial risk of disease spread. In contrast, CWD prevalence remains lower in hunted deer populations, but at a higher prevalence the disease competes with recreational hunting to reduce deer abundance.5 Synthesis and applications. Uncertainty about density‐ or frequency‐dependent transmission hinders predictions about the long‐term impacts of chronic wasting disease on cervid populations and the development of appropriate management strategies. An adaptive management strategy using computer modelling coupled with experimental management and monitoring can be used to test model predictions, identify the likely mode of disease transmission, and evaluate the risks of alternative management responses.

SPATIAL EPIDEMIOLOGY OF CHRONIC WASTING DISEASE IN WISCONSIN WHITE-TAILED DEER

Chronic wasting disease (CWD) is a fatal, emerging disease of cervids associated with transmissible protease-resistant prion proteins. The potential for CWD to cause dramatic declines in deer and elk populations and perceived human health risks associated with consuming CWD-contaminated venison have led wildlife agencies to embark on extensive CWD control programs, typically involving culling to reduce deer populations. We characterized the spatial distribution of CWD in white-tailed deer (Odocoileus virginianus) in Wisconsin to facilitate CWD management. We found that CWD prevalence declined with distance from a central location, was locally correlated at a scale of 3.6 km, and was correlated with deer habitat abundance. The latter result is consistent with patterns expected for a positive relationship between density and prevalence of CWD. We recommend management activities focused on culling in geographic areas with high prevalence to have the greatest probability of removing infected individuals. Further research is needed to elucidate the factors involved in CWD spread and infection rates, especially the role of density-dependent transmission.

Chronic Wasting Disease in Free-Ranging Wisconsin White-Tailed Deer

Three White-tailed Deer shot within 5 km during the 2001 hunting season in Wisconsin tested positive for chronic wasting disease, a prion disease of cervids. Subsequent sampling within 18 km showed a 3% prevalence (n=476). This discovery represents an important range extension for chronic wasting disease into the eastern United States.

Spatial and temporal patterns of chronic wasting disease: fine- scale mapping of a wildlife epidemic in Wisconsin

Emerging infectious diseases threaten wildlife populations and human health. Understanding the spatial distributions of these new diseases is important for disease management and policy makers; however, the data are complicated by heterogeneities across host classes, sampling variance, sampling biases, and the space-time epidemic process. Ignoring these issues can lead to false conclusions or obscure important patterns in the data, such as spatial variation in disease prevalence. Here, we applied hierarchical Bayesian disease mapping methods to account for risk factors and to estimate spatial and temporal patterns of infection by chronic wasting disease (CWD) in white-tailed deer (Odocoileus virginianus) of Wisconsin, U.S.A. We found significant heterogeneities for infection due to age, sex, and spatial location. Infection probability increased with age for all young deer, increased with age faster for young males, and then declined for some older animals, as expected from disease-associated mortality and age-related changes in infection risk. We found that disease prevalence was clustered in a central location, as expected under a simple spatial epidemic process where disease prevalence should increase with time and expand spatially. However, we could not detect any consistent temporal or spatiotemporal trends in CWD prevalence. Estimates of the temporal trend indicated that prevalence may have decreased or increased with nearly equal posterior probability, and the model without temporal or spatiotemporal effects was nearly equivalent to models with these effects based on deviance information criteria. For maximum interpretability of the role of location as a disease risk factor, we used the technique of direct standardization for prevalence mapping, which we develop and describe. These mapping results allow disease management actions to be employed with reference to the estimated spatial distribution of the disease and to those host classes most at risk. Future wildlife epidemiology studies should employ hierarchical Bayesian methods to smooth estimated quantities across space and time, account for heterogeneities, and then report disease rates based on an appropriate standardization.

Dynamics of a Harvested Bobcat Population in Oklahoma

Demographic parameters of bobcats (Felis rufus) in Oklahoma were determined in order to evaluate population status. A maximum density of one adult bobcat per 11.0 km2 was estimated for the Ouachita National Forest in southeastern Oklahoma assuming complete intersexual home-range overlap and no unoccupied areas. Statewide population trends determined by scent-station surveys declined (P = 0.004) from 1977 to 1981, with a finite rate of increase (X) of 0.89/year. Sex and age structure and reproductive rates were determined from examination of 553 carcasses and/or skulls. Sex ratios did not differ (P = 0.16) from an expected 50:50 ratio. The age structure of the harvest was 26% juveniles, 32% yearlings, and 43% adults. Pregnancy rate of yearling females (46%) was lower (P < 0.001) than in adults (92%). Yearling pregnancy rate was further reduced following a major drought which probably caused the observed decline in prey abundance on the Ouachita National Forest. Mean in utero litter sizes of yearlings and adults were 2.3 and 2.7 kittens/litter, respectively. Estimates of adult survival rate ranged from 0.53 to 0.66; juvenile survival was 0.30. Harvest was the sole source of non-study-related mortality of radio-collared bobcats. Continued harvest of already low density bobcat populations may further depress the populations and result in local extirpations. Reduction of harvest during periods of negative rates of increase is recommended. J. WILDL. MANAGE. 49(2):283-292 The economic value of bobcat pelts has increased dramatically during the last decade. The 1970-71 nationwide average price of bobcat pelts was

Deer density and disease prevalence influence transmission of chronic wasting disease in white‐tailed deer

12 (Deems and Pursley 1978). In Oklahoma, the average price paid for bobcat pelts increased from

Effects of Earn-a-Buck and Special Antlerless-Only Seasons on Wisconsin's Deer Harvests

12 in 1974-75 to

Reproductive ecology of eastern wild turkeys in southwestern Wisconsin

67 in 1980-81 (Day 1978; Okla. Dep. Wildl. Conserv., unpubl, rep., Fed. Aid Proj. W-82-R-20, 1982). Increased value of pelts has resulted in increased harvest in Oklahoma as well as nationally. The 1974-75 estimated harvest of bobcats in Oklahoma was 1,458 compared to 2,782 in 1980-81 (Okla. Dep. Wildl. Conserv., unpubl. reps., Fed. Aid Proj. W-82-R-19, 1980, and W-82-R-20, 1982). The reported nationwide harvest in 1970-71 was 10,822 (Deems and Pursley 1978) but increased to 86,998 in 1980-81 (U.S. Fish and Wildl. Serv. unpubl. rep., Amendment to Appendix II of the CITES, 1982). A better understanding of bobcat population dynamics is needed to evaluate the impact of increased harvest pressure. Crowe (1975a) presented a model of an exploited bobcat population in Wyoming. He assumed a non-age-related pregnancy rate of 100% and estimated the annual adult survival rate as 67%. This may have been biased because his sample was drawn from an apparently declining population (Caughley 1966). The purpose of the present study was to quantify demographic parameters of bobcats in Oklahoma, population density and trend, sex and age composition, and rates of reproduction and mortality, and to evaluate population status and assess the effects of exploitation. The effects of fluctuations in food availability on age-specific reproductive parameters were also examined. This study would not have been possible without the advice and guidance of J. H. Shaw, F. Schitoskey, P. A. Vohs, J. M. Gray, and G. A. Bukenhofer. Field assistance provided by M. E. Stewart, M. E. Wagner, and L. A. Ashford is greatly appreciated. C. D. Clubb, T. G. Clubb, and D. C. Clubb assisted with trapping. W. D. Warde assisted with statistical analyses. I am grateful for the assistance of the pilots and staff of the Poteau Flying Service. Special thanks are due to R. T. Hatcher for providing unpublished data from the Okla. Dep. of Wildl. Conserv. C. J. Brand reviewed an earlier draft of this manuscript. This study was funded by a contribution from Okla. Fed. Aid to Wildl. Restoration Proj. W-129-R through the Okla. Coop. Wildl. Res. Unit. Cooperators of the Okla. Coop. Wildl. Res. Unit include Oklahoma State Univ., Okla. Dep. of Wildl. Conserv., U.S. Fish and Wildl. Serv., and Wildl. Manage. Inst.

Transmission of Chronic Wasting Disease in Wisconsin White-Tailed Deer: Implications for Disease Spread and Management

Host-parasite dynamics and strategies for managing infectious diseases of wildlife depend on the functional relationship between disease transmission rates and host density. However, the disease transmission function is rarely known for free-living wildlife, leading to uncertainty regarding the impacts of diseases on host populations and effective control actions. We evaluated the influence of deer density, landscape features, and soil clay content on transmission of chronic wasting disease (CWD) in young (<2-year-old) white-tailed deer (Odocoileus virginianus) in south-central Wisconsin, USA. We evaluated how frequency-dependent, density-dependent, and intermediate transmission models predicted CWD incidence rates in harvested yearling deer. An intermediate transmission model, incorporating both disease prevalence and density of infected deer, performed better than simple density- and frequency-dependent models. Our results indicate a combination of social structure, non-linear relationships between infectious contact and deer density, and distribution of disease among groups are important factors driving CWD infection in young deer. The landscape covariates % deciduous forest cover and forest edge density also were positively associated with infection rates, but soil clay content had no measurable influences on CWD transmission. Lack of strong density-dependent transmission rates indicates that controlling CWD by reducing deer density will be difficult. The consequences of non-linear disease transmission and aggregation of disease on cervid populations deserves further consideration.

Wild turkey population dynamics in southwestern Wisconsin

Abstract Effective management of wildlife populations often requires motivating hunters to harvest sufficient numbers of animals of prescribed sex and age classes to meet management goals. For cervids, it is convenient to design harvest regulations relative to presence (male) or absence (young and female) of antlers because harvest of females has a larger effect on population growth. We used regression techniques to evaluate effects of 2 supplemental hunting programs based on additional days of hunting opportunity and an additional incentive used to complement additional days on harvest of antlered and antlerless deer in Wisconsin, USA. Earn-a-buck regulations, an incentive-based program that requires hunters to register an antlerless deer before being authorized to harvest an antlered deer, were associated with an average increase of 2.04 deer/km2 in antlerless harvest and a 0.60 deer/km2 decrease in harvest of antlered deer. Providing more opportunity for hunting of antlerless deer in the form of 4- and 8-day supplemental firearm seasons was associated with 1.10 deer/km2 and 1.32 deer/km2 increases, respectively, in antlerless harvest with trivial (0.02 deer/km2 and 0.09 deer/km2) decreases in harvests of antlered deer. Our analysis suggests that extra days of hunting opportunity coupled with the earn-a-buck incentive was 56–88% more effective at increasing antlerless harvest relative to additional days of hunting without the incentive. Use of the earn-a-buck incentive resulted in decreased harvest of antlered deer and was disliked by many hunters. Quantifying these relationships is important for helping managers predict the costs and benefits of various hunting programs.